1. Field of the Invention
The present invention relates to a display apparatus including mounting an active matrix type display panel.
2. Description of Related Art
Nowadays, an electroluminescent display apparatus (hereinafter referred to as “the EL display apparatus”) incorporating a display panel that uses organic electroluminescent devices (hereinafter referred to as “EL devices”) as luminescent devices carrying pixels has been attracting attention. A simple matrix drive type and an active matrix drive type are known as the driving scheme for the display panel of the EL display apparatus. The active matrix drive type EL display apparatus is advantageous in that it consumes less power and incurs less cross talk among pixels, as compared with the simple matrix type, making it particularly suitable for a large-screen display or a high-definition display.
FIG. 1 schematically shows the construction of an active matrix drive type EL display apparatus.
The EL display apparatus shown in FIG. 1 is constructed of a display panel 10 and a drive unit 100 for driving the display panel 10 in response to a video signal VL.
The display panel 10 has an anode power bus line 16, a cathode power bus line 17, scanning lines or scanning electrodes A1 through An for n horizontal scanning lines of one screen, and m data lines or data electrodes B1 through Bm disposed to cross the scanning lines. A power potential Vc is applied to the anode power bus line 16, while a ground potential GND is applied to the cathode power bus line 17. Furthermore, EL elements E1,1 through En,m carrying the pixels are formed at the intersections of the scanning lines A1 through An and the data lines B1 through Bm in the display panel 10.
FIG. 2 shows an example of the internal construction of an EL unit E formed at the intersection of a scanning line A and a data line B.
Referring to FIG. 2, the scanning line A is connected to a gate G of a field effect transistor (FET) 11 for selecting scanning lines, a data line B being connected to a drain D thereof. A gate G of a FET 12 acting as a light emission drive transistor is connected to a source S of the FET 11. The power potential Vc is applied to a source S of the FET 12 via the anode power bus line 16, and a capacitor 13 is connected between the gate G and the source S. Furthermore, an anode end of an EL device 15 is connected to a drain D of the FET 12. The ground potential GND is applied to the cathode end of the EL device 15 via the cathode power bus line 17.
The drive unit 100 selectively applies scanning pulses to the scanning lines A1 through An of the display panel 10 in sequence. In synchronization with the application timings of the scanning pulses, the drive unit 100 also generates pixel data pulses DP1 through DPm on the basis of the video signal VL corresponding to each of the horizontal scanning lines, and applies the generated pulses to the data lines B1 through Bm. Each of the pixel data pulses DP has a pulse voltage based on the luminance level indicated by the video signal VL. Pixel data is written to the EL devices connected to the scanning line A to which a scanning pulse is applied. The FET 11 in the EL unit E to which the pixel data is written turns ON in response to the scanning pulse, and applies the pixel data pulse DP supplied via the data line B to the gate G of the FET 12 and the capacitor 13. The FET 12 produces a light emission drive current based on the pulse voltage of the pixel data pulse DP and applies the produced current to the EL device 15. With the light emission drive current, the EL device 15 emits light at a luminance based on the pulse voltage of the pixel data pulse DP. Meanwhile, the capacitor 13 is charged by the pulse voltage of the pixel data pulse DP. The charging operation maintains the capacitor 13 at the voltage level corresponding to the luminance level indicated by the video signal VL, causing pixel data to be written. When the pixel data has been written, the FET 11 turns OFF to stop the supply of the pixel data pulse DP to the gate G of the FET 12. However, the voltage held at the capacitor 13 mentioned above continues to be applied to the gate G of the FET 12, so that the FET 12 continues to supply the light emission drive current to the EL device 15. This means that, even after the writing of the pixel data has been completed, the EL device 15 continues to emit light at a luminance based on the luminance level indicated by the video signal VL.
On the other hand, the characteristics of the FET 11, the FET 12, and the EL device 15 vary according to temperature or with time. This has been posing a problem in that, if, for example, an ambient temperature changes, then the light emission drive current passing through the EL device 15 does not reach a desired current value, so that the EL device 15 cannot emit light at a proper luminance based on a received video signal.